The present-invention-relates to a process for preparing and isolating 9-deoxo-9(Z)-hydroxyimino-erythromycin A (referred to hereinbelow as 9(Z)-erythromycin oxime or 9(Z)-oxime) from the corresponding E isomer.
The present invention lies in the field of macrolide antibiotics of erythromycin type and relates more particularly to their aza-macrolide derivatives which are the subject of patent application EP 508 699 and correspond to the following general formula: 
in which R represents a hydrogen atom, a C1-C10 alkyl group, a C2-C10 alkenyl group or a C6-C12 arylsulphonyl group, which may be substituted.
These compounds are obtained from erythromycin and their synthesis involves two major steps:
the creation of the 8a-azalide macrocycle from 9(E)-erythromycin oxime isomerized into the corresponding 9(Z)-oxime, which then undergoes a stereospecific Beckmann rearrangement, and
the modification of the xe2x80x9ccladinosexe2x80x9d group in position 4, which consists in converting the 4(S)-OH into 4(R) -NH2.
The present invention relates more particularly to the first step of this synthesis and its subject is a new process for isomerizing 9(E)-erythromycin oxime and isolating the resulting 9(Z)-oxime isomer, which can be illustrated as follows: 
This isomerization step is, in particular, the subject of patent application EP 503 949, in which the 9(Z)-oxime of formula (I) is obtained by treating the E isomer of formula (II) with a base, preferably an alkali metal hydroxide such as lithium hydroxide, in a protic or aprotic solvent which is preferably ethanol. The residue obtained after evaporating off the solvent is taken up in ethyl acetate and an aqueous solution which is then re-extracted with ethyl acetate to give a crude product containing a mixture of oximes. The crude mixture of oximes is then taken up in methylene chloride, then filtered. The solid obtained is then taken up in ethyl acetate and a non-solvent (nitromethane) and then crystallized or purified in ethyl acetate by successive steps of precipitation with methylene chloride, and filtrations.
As it turns out, the current conditions cannot be extrapolated to the industrial scale.
The reason for this is that this process involves steps of concentrating to dryness of the reaction mass in ethanol and of that in ethyl acetate.
It also involves the use of chlorinated solvents that are undesirable in terms of environmental protection.
Finally, the product isolated still contains the (E) isomer and needs to be taken up several times in a medium containing ethyl acetate and methylene chloride in order to crystallize (by xe2x80x9cbeatingxe2x80x9d) the desired (Z) isomer and to isolate it in an acceptable isomeric purity.
The aim of the present invention is to provide an efficient alternative to the known process which makes it possible to overcome the abovementioned drawbacks.
The aim of the invention is thus to provide a simplified process, which is easy to carry out on the industrial scale and which gives the 9(Z)-oxime in a satisfactory isomeric purity.
The aim of the invention is, in particular, to avoid the use of chlorinated solvents, that are environmentally harmful, as well as the laborious purification by xe2x80x9cbeatingxe2x80x9d in an ethyl acetate/methylene chloride mixture.
A subject of the present invention is a process for preparing 9-deoxo-9(Z)-hydroxyiminoerythromycin A corresponding to formula (I) below: 
successively comprising the steps consisting in:
reacting in water 9-deoxo-9(E)-hydroxyimino-erythromycin A corresponding to formula (II) below: 
with a base,
acidifying the reaction mixture to a pH of between 9 and 11,
adding to the said mixture an organic solvent;
optionally concentrating under vacuum the resulting organic phase;
isolating the desired 9(Z)-erythromycin oxime.
According to a preferred variant of the invention, for the reaction of the 9(E)-oxime of formula (II), an organic solvent of dialkyl ketone type, in particular acetone, is added to the water.
The inventors have demonstrated, unexpectedly, that 9(E)-erythromycin oxime suspended in water, optionally with the addition of a solvent of the dialkyl ketone type, can be isomerized with a base, without the presence of an alcoholic solvent, followed by directly extracting, after neutralization of the salt, the desired isomer from the reaction suspension and isolating it in a satisfactory purity.
They have thus demonstrated that the desired (Z) isomer can be obtained by adding to the reaction suspension an organic solvent such as ethyl acetate allowing it to be crystallized without addition of another solvent to the medium. Depending on the case, this medium may be capable of forming an insoluble or sparingly soluble solvate with the 9(Z)-oxime. It does not require any subsequent crystallizations.
According to one embodiment, the invention thus covers the use, after isomerization in water, of any organic solvent capable of inducing the crystallization of the 9(Z)-oxime, in particular by concentration, in the said solvent, while the (E) isomer remains mainly dissolved in the medium.
According to a preferred variant, it is the actual isomerization which is carried out in water to which is added an organic solvent of dialkyl ketone type capable of forming a crystallizable solvate with the (Z) isomer as indicated above.
The inventors have thus developed a simplified process for dispensing with the precipitation with methylene chloride as well as the multi-step purification required in the known process.
The inventors have also demonstrated that the mother liquors (containing a mixture of the E and Z isomers) collected after isolation of the crystallized (Z) isomer can advantageously be recycled by reforming an aqueous suspension of the mixture of isomers they contain after removal of most of the organic solvent present.
The process according to the invention will be described in greater detail below.
This process consists firstly in treating 9(E)-erythromycin oxime, suspended in water, with a base which is preferably water-soluble.
According to a preferred variant of the invention, the 9(E)-oxime is reacted with the base, in an aqueous medium formed of water mixed with an organic solvent of dialkyl ketone type and advantageously capable of forming a crystallizable solvate with the 9(Z)-oxime, while the (E) isomer remains mainly in solution. The solvent of dialkyl ketone type is preferentially chosen from dialkyl ketones containing from 3 to 10 carbon atoms and it is typically acetone.
The base is then added to give rise to the isomerization reaction. Examples of bases which may be mentioned are alkali metal or alkaline-earth metal hydroxides, ammoniums, carbonates and alkoxides. This base preferably consists of lithium hydroxide or sodium hydroxide.
The base is used in an amount preferably of between 1 and 10 equivalents, preferably 2 molar equivalents relative to the 9(E)-oxime.
The addition of the base to the 9(E)-oxime leads to its deprotonation and makes it possible to reach the equilibrium conditions with the (Z) isomer.
The pH of the reaction medium is generally between 11.5 and 14.
The subsequent treatment applied to the mixture makes it possible to shift this equilibrium and preferentially to isolate the 9(Z)-oxime in the form of solvate.
The reaction is generally carried out under an inert atmosphere. The Z/E ratio is temperature- dependent and the reaction is preferably carried out at a temperature of between 100 and 25xc2x0 C., more preferably in the region of 20xc2x0 C.
The reaction medium is preferably stirred for 6 to 24 hours.
The desired (Z) isomer is then extracted with an organic solvent, in particular with ethyl acetate or another equivalent solvent.
To do this, the reaction mixture is first acidified to a pH preferably of between 9 and 11, even more preferably to a pH of about 9.5-10. For this, hydrochloric acid, acetic acid or sodium bicarbonate is preferably used.
To carry out this acidification step, the said mixture is preferably cooled to a temperature below 20xc2x0 C., and more preferably to a temperature of about An organic solvent is then added to the reaction medium in order to induce the crystallization of the desired (Z) isomer.
When the isomerization reaction is conducted in water, according to one preferred embodiment of the invention, ethyl acetate or other solvents which have equivalent properties in terms of crystallization of the (Z) isomer is used.
The expression xe2x80x9csolvents which have equivalent properties of crystallizing the (Z) isomerxe2x80x9d means any solvent capable of inducing crystallization of the 9(Z)-oxime, in particular by concentrating the organic extraction phase, while the (E) isomer remains mainly in solution.
Specifically, according to this embodiment of the invention, it is thought that the (Z) isomer can be extracted directly from the reaction medium and can crystallize by concentration in the organic extraction solvent. When the isomerization reaction is carried out in a water/dialkyl ketone mixture, it is the solvate of the (Z) oxime with the dialkyl ketone which precipitates at the end of neutralization. The use of an organic solvent such as ethyl acetate or methyl butyl ether makes it possible to improve the Z/E ratio in favour of the desired (Z) isomer, during the filtration of the solvate with the dialkyl ketone. Moreover, an ester such as ethyl acetate also makes it possible to improve the subsequent drying of the (Z) oxime by promoting the removal of the solvent of dialkyl ketone type.
During the extraction step, the temperature of the reaction medium is preferably returned to room temperature (about 25-30xc2x0 C.) which facilitates the separation of the phases by settling.
After separation of the phases by settling, the aqueous phase is preferably re-extracted under the abovementioned conditions.
Where appropriate, the organic extraction phases are combined and then concentrated under vacuum in order to bring about crystallization of the desired (Z) isomer in the medium.
The temperature of the reaction medium is preferably maintained below 35xc2x0 C. during this concentration operation and is preferably carried out for several hours (for about 4 to 5 hours).
The desired (Z) isomer is then isolated by filtration. For this, the reaction mass is preferably maintained between 10 and 25xc2x0 C., preferably cooled to a temperature of about 10xc2x0 C.
The (Z) isomer is recovered in a Z/E ratio of greater than 90/10, typically between 93/7 and 98/2.
According to the invention, the mother liquors collected after filtration, which essentially contain the 9(E)-oxime, can be reprocessed as indicated above.
In this case, most of the ethyl acetate or other extraction solvent is distilled off under vacuum on a tail of water, until only 3 to 4% remains, for example.
A solvent of dialkyl ketone type and base are then added, if appropriate, as indicated above, in order to carry out a new isomerization of the 9(E)-oxime present, followed by isolation of the 9(Z)-oxime formed under the conditions indicated above.
Depending on the extraction solvent, a larger amount of base may need to be introduced on account of a possible saponification of the solvent.
The process according to the present invention has the advantage of using only one extraction solvent, which is generally not a chlorinated solvent, and of not requiring multiple repeats in order to obtain the crystallization of the desired isomer. It can be carried out easily in industrial terms.
The process according to the invention is illustrated below by examples which should not be considered as limiting.